The present invention relates generally to coating compositions suitable for coating glass and glass products. More particularly, the invention relates to radiation-curable acrylate coating compositions for glass optical fibers.
A relatively recent innovation in the field of telecommunications is the development of glass optical fibers. These fibers, when properly protected by suitable coatings and cabling, have the capability of carrying large amounts of information over long distances with very low signal attenuation.
As is well known, however, protective coatings must be applied to these glass telecommunications fibers at the time of manufacture, both to preserve the inherent strength of the fibers and to protect them from certain types of bending which can induce signal loss in telecommunication cables. More specifically, the coatings applied to the fibers must enable them to resist short radius bending (so-called microbending) which can arise when the coated or cabled fiber is subjected to physical stress and/or changes in temperature in use.
In combination, these requirements dictate that the coating applied to the optical fiber have both substantial toughness and yet be soft enough to distribute transverse strain applied to the fiber. A wide variety of organic coating materials designed to meet these needs have been proposed for use as optical fiber coatings, including but not being limited to silicone polymers, silicone oils, lacquers, and urethane and/or acrylate polymers.
An additional requirement for optical fiber coating materials derives from the fact that very high optical fiber drawing speeds are now being employed in the industry for reasons of manufacturing efficiency. For economic production, therefore, a glass coating material is needed which can be rapidly applied to and cured on the surface of the optical fiber. Currently, the preferred coating materials for rapid optic fiber production are radiation-curable coating formulations which can be very rapidly cured without the need for solvent volatilization or heating, simply by exposing the fibers coated with the uncured resin coating material to intense radiation.
Among the radiation-curable coating formulations presently used for the application of protective coatings to glass optical fibers are radiation curable acrylate materials. These are typically ultraviolet-curable oligomers or polymers, available in liquid form, which exhibit good softness over a very broad temperature range, good tensile strength and toughness, and rapid UV curing characteristics.
Preferred examples of these UV-curable acrylate compositions are reported in published European patent applications EP 0204160 and EP 0204161. The compositions therein described, loosely termed UV-curable polyurethane acrylates in the art, are based on resins more specifically designated in these applications as acrylate-terminated polyurethane, polyurea, or polyurethane/polyurea oligomers.
It is of course important that the resin formulations selected for application to these optical fibers maintain properties over an extended period of time. Thus the coatings should not exhibit significant changes in properties, and especially in their elastic properties, with prolonged exposure to ambient temperature variations and other environmental conditions. Many of the older optical fiber coating materials exhibited substantial variations in properties when exposed to environmental changes, these variations being attributable to the chemical structure, purity, and/or the degree of cure of the coatings. To counteract these effects, it has been conventional to employ various additives such as antioxidants, UV-stabilizers, and antihydrolysis agents which help to stabilize the cured coatings against changes in composition and structure.
The type of additive employed of course depends upon the coating system being modified. Thus it is not readily predictable whether an additive having demonstrated effectiveness in one resin system will exhibit helpful or harmful effects when employed in a different resin system. In the case of polyurethane acrylates such as presently utilized for optical fibers, additives such as hindered piperidine derivatives have been used in the past. See, for example, "Weathering of UV Cured Coatings," L. R. Gatechair, UV Curing: Science and Technology, S. P. Pappas, Ed., Vol. 2, Chapter 7 (1985). However, other additives have not been used because of concerns about their effects on the cured and uncured resins. For example, it is known that antioxidant additives can interfere with the free radical curing process relied upon for the rapid curing of these resins.
When adding stabilizing constituents to UV-curable resin formulations for use as optical fiber coatings, several key properties of the resins must remain substantially unaffected. The additives must not reduce the curing speed of the resin used to apply the coating, must not undesirably affect the shelf life of the uncured resin formulation, and must not affect the short or long term elastic properties of the cured coating.
Presently available resins of the kind described in the aforementioned published European patent applications, some of which are currently used to provide high-elasticity, low-modulus, primary protective coatings on optical fibers, generally exhibit all of the required properties of rapid cure, low elastic modulus, and high toughness. However, it has recently been determined that at least some of these formulations do not exhibit the requisite thermal stability. Thus sample materials of these types have recently been evaluated which exhibit readily observable deterioration on brief exposure to elevated temperatures.
While the precise mechanism giving rise to the thermal instability observed in these coatings has no been identified, it has been observed that significant weight loss in the radiation-cured resins can be observed at heating temperatures as low as 85.degree. C., with substantial losses occurring after even a relatively brief exposure to temperatures as low as 130.degree. C. Moreover there is reason to believe that thermal degradation also occurs, although more slowly, at even lower temperatures such as may be encountered by optical fibers in actual use. This is deemed unacceptable since gas or liquid by-products of the degradation reaction could potentially cause fiber attenuation problems, as the result of unpredictable stress generation by these by-products when trapped in optical cables.
It is a principal object of the present invention to provide a solution to the problem of cured coating thermal instability in polyurethane and/or polyurea acrylate coatings of the presently used type above described.
It is a further object of the invention to provide stabilized polyurethane or polyurea coatings which, after curing, do not exhibit the rapid thermal degradation of the known coatings of this type.
Other objects and advantages of the invention will become apparent from the following description thereof.